<!DOCTYPE html>
<html>
  <body onload="onLoad()">
    <textarea></textarea>
    <div id="start"></div>
    <div id="end"></div>
    <script>
      function onLoad() {
        var t = document.querySelector("textarea");
        t.value = "aaa\nbbb";
        t.value = "aaa\nbbb\n";
        t.value += "X";
        document.getElementById("start").textContent = t.selectionStart;
        document.getElementById("end").textContent = t.selectionEnd;
      }
    </script>
  </body>
</html>
